When you bid on an eBay auction, the proxy bidding system takes your maximum bid and applies it one increment at a time against other bids.
So
The item has an asking bid of $10
You bid $100.
The item opens to you at $10.
I bid $11.
The item is still yours now at $12.
And so on until someone bids $101.
Now your bids will no longer be considered.
If what you mean is that you bid before you noticed that the item was used, and now wish to lower your maximum bid, go here:
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow
retract your too high bid, and insert the new correct amount.
Bid retractions show on your feedback page.